This invention relates to gas turbine engine nozzles, and more particularly to nozzle constructions for jet noise suppression.
Jet noise is created by the turbulent mixing of high velocity engine gases that emanate from the downstream end of a modern gas turbine. The turbulent mixing occurs between the high velocity gases and between the high velocity gases and ambient flow. The high velocity exhaust gases are typically a mixture of two sourcesxe2x80x94the hot gases resulting from the combustion within the engine core flow (primary source) and cooler air discharged from fan bypass ducts (secondary source). The velocity of the core flow is on the order of 1600 ft/sec, while the velocity of the fan bypass flow is on the order of 1000 ft/sec. The velocity gradient that exists at the different interfaces or shear regions, namely between the core and the fan exhaust flows, between the fan exhaust flow and ambient, and between the core flow and ambient, results in flow disturbances. These flow disturbances or turbulence results in jet noise. The turbulent flow in the shear regions between the high velocity gases and the ambient air produce a significant component of the high levels of noise that are objectionable for aircraft operation from commercial airports.
Due to the adverse impact noise has on the environment, many countries and airports have imposed increasingly strict noise reduction criteria on aircraft. In the United States, the Federal Aviation Administration (FM) has imposed strict noise reduction limits on aircraft that are currently in use. In addition, the restrictions imposed by various airports range from financial penalties and schedule restrictions to an outright ban on the use of the aircraft. An effective and efficient noise reduction solution is necessary since these restrictions would severely cut short the useful life for certain types of aircraft that commercial airlines are currently using.
Turbofan engines are categorized as either low bypass ratio or high bypass ratio, based on the ratio of bypass flow to core flow. Jet noise is a well-known problem with low bypass ratio engines. In the low bypass ratio jet engines, the exhaust gases emanating from the core and fan bypass ducts usually mix before they exit the engine""s exhaust nozzle, where they form a high speed plume. The plume rips or shears against the slower ambient air as it rushes by creating flow turbulence and thus jet noise.
Typically, newer jet engines are high bypass ratio engines which have lower (but still significant) levels of jet noise than low bypass ratio engines. High bypass ratio engines usually have separate-flow exhaust systems. High bypass ratio engines have much larger fan flows, and overall larger total engine flow rates than the low bypass ratio engines. Thrust is obtained through larger mass flow rates, and lower jet velocities than low bypass ratio engines. Due to lower jet velocities, the level of jet noise is decreased in these high bypass ratio engines as compared to the low bypass ratio engines.
However, jet noise remains a problem for modern high bypass ratio engines especially during operation at high engine power levels. High engine power is typically associated with aircraft take-off scenarios when the engine produces a high thrust and results in high velocity exhaust air. The FAA imposes strict noise requirements at high power. Modern, high bypass ratio engines have to comply with the requirement to provide ever-higher thrusts to power new and growth versions of aircraft with increasing takeoff gross weight. As a result, the modern, high bypass ratio engines operate at higher jet temperatures and pressure ratios and generate higher jet velocities and thus higher jet noise levels than earlier models of high bypass ratio engines.
In the prior art of jet noise suppression, different structures have been devised to reduce noise. For example, a lobed mixer concept has been used in the past for the low bypass ratio engines which have a long duct, common flow exhaust system such as those used in Pratt and Whitney""s JT8D engine family.
Examples of such noise suppression structures are found in U.S. Pat. No. 4,401,269 to Eiler and U.S. Pat. No. 5,638,675 to Zysman et al, both assigned to the assignee of the present application, which disclose lobed mixers for a gas turbine engine. The lobed mixer includes axially and radially extending chutes. The chutes act as gas conduits whereby relatively cool, low velocity fan air is directed into the chutes and in turn into the hot, higher velocity core gas flow. The lobed mixer thus increases the mixing of the core and fan bypass gases.
While the long duct, common flow exhaust systems of the prior art, as represented by the exhaust nozzles of the JT8D engine family, and the ""269 and ""675 patents, have met with great commercial acceptance in the aerospace industry, the assignees of the present invention are constantly looking to improve the exhaust nozzle system of gas turbine engines, especially during operation of the engines at high power levels. Studies and nozzle configurations including tab concepts have been proposed to achieve mixing.
Even though tabbed mixing devices are generally known, these devices have been unsuitable for jet engine applications. Typically, tabs disposed in fluid flow streams are known to increase noise because the tabs provide cross-stream mixing over a wide flow area within the entire fluid flow stream. The tabs create pairs of oppositely rotating vortices, which in turn generate noise.
Further, not only would typical tabbed mixing devices adversely impact jet noise, they would also adversely impact engine thrust or performance. The angular orientation of the tabs would introduce unacceptable thrust losses due to the high degree of penetration of typical tabs into the fluid flow stream. The tabs would extract useful energy from the flow stream and would cause a significant thrust loss to the engine.
A primary object of the present invention is the provision of enhanced jet noise suppression, especially during engine operation at high power levels.
A further object of the present invention is the provision of enhanced jet noise suppression without the addition of appreciable thrust losses.
Another object of the present invention is the provision of a jet noise suppression system, which does not increase high frequency noise.
Another object of the present invention is the provision of a jet noise suppression system that minimizes the addition of weight.
According to the present invention, a gas turbine engine jet noise suppressor which does not appreciably adversely impact engine thrust and performance includes two concentric nozzles with associated flow streams and an arrangement of nozzle tabs disposed in at least one of the nozzles, that are directed and extend in a radially inward direction for increasing the effectiveness of the mixing process at the interface between exhaust gas flow streams and the ambient air. The nozzle tabs are trapezoidal in shape and are disposed circumferentially at the exit of an exhaust nozzle. In a preferred embodiment of the present invention, each tab is spaced apart from adjacent tabs and is directed radially inwardly into the exhaust nozzle with a predetermined angular relationship with respect to the flow stream.
The present invention suppresses jet noise with minimal impact to engine thrust and performance. The tabs of the present invention have relatively small angles of protrusion into the engine flow. The tabs are angled inwardly only a small amount, with angles ranging between 5 to 15xc2x0, commensurate with the interface between the core and fan flow streams which spans a relatively small radial distance. The present invention creates vortices or swirling motion at the interface between the distinguishable flow streams of the exhaust nozzles and ambient air. These vortices pull the flow at the interface of the core and fan flow streams into mixing engagement. The vortices however do not appreciably adversely impact engine thrust and performance as they are small, acting primarily on the narrow interface flow area between the engine flow streams. The interface mixing provided by the present invention shortens the effective length of the jet plume thereby reducing noise generation in the nozzle.
Further, the present invention has only a minimal impact on the weight of the gas turbine engine because the tabs are relatively small as compared to the diameter of the nozzle. In a preferred embodiment, the ratio of the length of the tabs to the diameter of the nozzle is approximately 0.04. In addition, the ratio of the height of the tab protruding into the flow stream to the diameter of the nozzle is approximately 0.006. The height and number of tabs is a function of nozzle geometry.
The foregoing and other objects, features and advantages of the present invention will become more apparent in the following detailed description of the best mode for carrying out the invention and from the accompanying drawings that illustrate an embodiment of the invention.